The present invention relates to a display sheet, which can change states to provide a viewable image.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically written areas to carry ticketing or financial information, however magnetically written data is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbookxe2x80x9d, each sheet can be individually addressed. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal material that ceases to present an image when de-energized.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Said structure has the capacity of maintaining one of the given states in the absence of an electric field.
U.S. Pat. No. 3,816,786 discloses a layer of encapsulated cholesteric liquid crystal responsive to an electric field. The conductors in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one conductor must be light absorbing and it is suggested that the light absorbing conductor be prepared from paints containing conductive material such as carbon.
U.S. Pat. No. 5,289,301 discusses forming a conductive layer over a liquid crystal coating to form a second conductor. The description of the preferred embodiment discloses Indium-Tin-Oxide (ITO) over a liquid crystal dispersion to create a transparent conductor.
Cholesteric materials require one of the two conductors to be light absorbing and conductive. Materials have been proposed for the application including carbon or metal oxides to create a black and conductive surface for polymer dispersed cholesteric liquid crystal materials. Such coatings often back scatter light. Moreover, because there is inactive material between the conductors, it would be desirable to maximize the use of the inactive material.
It is an object of this invention to provide a highly efficient light absorbing layer.
It is a further object of the invention to provide a way of increasing the active area driven by opposed conductors.
It is a still further object of this invention to provide the light absorbing layer with minimal effect on the driving voltage.
These objects are achieved in a display sheet having polymer dispersed liquid crystals, comprising:
a) a substrate;
b) a state changing layer disposed over the substrate and defining first and second surfaces, such state changing layer having the polymer dispersed liquid crystals having first and second optical states, which can change state;
c) a first conductor disposed over the first surface of the state changing layer;
d) a second conductor on the second surface of the state changing layer so that when an electrical field is applied between the first and second conductors, the liquid crystals change state; and
e) a nonconductive, field spreading layer having polymer dispersed sub-micron particles disposed between the state changing layer and the first conductor to provide a change of state in the state changing layer outside of areas between both conductors in response to a field applied between the first and second conductors which changes the state of the liquid crystals.
The present invention maximizes light absorption of the light absorbing layer, and improves on the active areas driven by opposed conductors. The structure of the field spreading layer minimizes additional voltage required for a thicker active materials coating.